Explosives
are classified as low or high explosives according to their rates of decomposition:
low explosives burn rapidly (or deflagrate), while high explosives undergo
detonation. No sharp distinction exists between low and high explosives,
because of the difficulties inherent in precisely observing and measuring
rapid decomposition.
The chemical decomposition of an explosive may take years, days, hours,
or a fraction of a second. The slower processes of decomposition take
place in storage and are of interest only from a stability standpoint.
Of more interest are the two rapid forms of decomposition, deflagration
and detonation.
The term "detonation" is used to describe an explosive phenomenon
whereby the decomposition is propagated by the explosive shockwave traversing
the explosive material. The shockwave front is capable of passing through
the high explosive material at great speeds, typically thousands of meters
per second.
Explosives usually have less potential energy than gasoline, but it is
the high rate of energy release that produces the blast pressure. TNT
has a detonation velocity of 6,940 m/s compared to 1,680 m/s for the detonation
of pentane in air, and the 0.34 m/s stoichiometric flame speed of gasoline
combustion in air.
Explosive force is released in a direction perpendicular to the surface
of the explosive. If the surface is cut or shaped, the explosive forces
can be focused directionally, and will produce a greater local effect.
This is known as a shaped charge.
In a low explosive, the decomposition is propagated by a flame front which
travels much more slowly through the explosive material.
The properties of the explosive indicate the class into which it falls.
In some cases explosives can be made to fall into either class by the
conditions under which they are initiated. In sufficiently massive quantities,
almost all low explosives can undergo true detonation like high explosives.
For convenience, low and high explosives may be differentiated by the
shipping and storage classes..